Edwards Aquifer: Past, Present and Future
Early Europeans first settled along perennial streams sustained by natural spring flows from the Edwards Aquifer (Maclay and Land, 1988). Substantial well discharge from the aquifer began in the late 1800's and and steadily increased from 101,900 ac-ft in 1934 to a record high of 542,400 ac-ft in 1989. The groundwater is used (figure 1) extensively for public water supply and agriculture, and accounted for, respectively, 56.6% and 30.1% of the total well discharge during 1981-90. The continual increase in well discharge (figure 2) has had an effect on the natural spring discharge, which comprises of 42.2% (1981-90) of the total withdrawal from the Edwards Aquifer. San Pedro and San Antonio Springs in San Antonio have become intermittent. Historically, the Comal Springs stopped flowing for two months in the summer of 1956. In 1984 and 1990, some of the higher Comal Springs ceased to flow and water levels (figure 3) in the index well dropped to within twelve feet of the 1956 record.
A major problem facing the Edwards Aquifer is the threat of overdrafting the average annual recharge (figure 4) and maintaining natural springflows. Accordingly, the Edwards Aquifer Authority (EAA) was authorized by the state legislature to develop, implement, and enforce a Critical Management Plan (CMP). The Cities of San Antonio, New Braunfels, and San Marcos have had to enforce Water Conservation Plan ordinances and water use disputes among aquifer users have become more frequent. In the most current case, the Sierra Club sued the Secretary of the Interior and the U.S. Fish and Wildlife Service (FWS) for failure to perform duties under the Endangered Species Act (ESA) and for injunctive relief. The U. S. District Court for the Western District of Texas reached a decision on January 30, 1993, and ordered FWS to determine required springflows, and the Texas Water Commission (TWC, now the Texas Natural Resource Conservation Commision or TNRCC) to prepare a plan assuring springflows will not drop below jeopardy levels. The court threatened additional orders if the State Legislature did not set up a regulatory system to limit withdrawals from the aquifer. In response, the 73rd Texas Legislature passed the Senate Bill No. 1477 to create the Edwards Aquifer Authority (EAA) and abolish the Edwards Underground Water District (EUWD), effective September 1, 1993. Governed by an appointed Board of Directors, the authority's primary function is to regulate the aquifer pumpage by limiting the long-term annual withdrawal at 400,000 ac-ft. The limiting amount is believed to be adequate to maintain the springflows at the Comal and the San Marcos, although the court opinion stated this level to be 200,000 ac-ft. Aquifer modeling on a monthly basis is a part of the effort to improve the understanding of the quantitative relationship among recharge, pumpage, springflow, and water levels. Such models will allow efficient and prudent management options to be explored without actually implementing the plan that would cause take or jeopardy of federally-listed species .